1. Field of the Invention
The instant invention relates to the regeneration of "spent" emulsions which are useful in liquid membrane separation processes. Regeneration is carried out by converting an impermeable species which is trapped in the internal phase of said emulsion into a permeable species by means of oxidation-reduction reaction which may be initiated by heat, light, or the absence of light. During such conversion, the emulsion is preferably agitated with a solvent which is immiscible with the external phase of the emulsion but miscible with said permeable species at conditions wherein the emulsion is not broken, i.e. the integrity of the external phase is maintained.
2. Description of the Prior Art
There has emerged a new field of technology involving liquid membrane extraction. Broadly, the principle involves surrounding either a fluid phase from which a species is to be extracted or a fluid phase in which a species is to be collected by a liquid membrane which is immiscible therewith. The surrounded fluid phase is the internal phase, of an emulsion or foam while the liquid membrane is the external phase. During the liquid membrane process, the emulsion or foam, i.e. the internal phase surrounded by the external phase, is contacted with another fluid from which or to which species permeate through the liquid membrane. Generally, both the fluid to be extracted and the fluid which is used to collect the species extracted from said fluid are liquids. The liquid membrane extraction process thus requires dispersing an emulsion as droplets in a liquid which is immiscible with the external phase of said emulsion at conditions wherein species in the internal phase of said emulsion may permeate through said external phase into said liquid or vice-versa without disruption of the liquid membrane. That is, the emulsion must be stable.
In an improvement of the liquid membrane process, the species is chemically converted to a nonpermeable species after it permeates the liquid membrane and is thus "trapped" in the phase to which it has permeated. Trapping promotes diffusion or transport through the liquid membrane and thus enables higher levels of extraction to occur.
In this improvement, the external phase (or liquid membrane) serves as an intermediary acting in the same way as an immiscible liquid in liquid-liquid extraction, except that it is constantly stripped of the species which it extracts.
Illustrative of the patents which have been issued in the field of development and application of liquid membrane technology include U.S. Pat. Nos. 3,389,078; 3,410,794; 3,617,546; 3,637,488; 3,650,091; 3,696,028; 3,719,590; 3,773,776; 3,740,315; 3,740,329; 3,779,907; 3,897,398; 3,942,527 and 3,969,265 and reissues thereof, each incorporated herein by reference.
It is appreciated by those skilled in the art that the external phase to function as a liquid membrane must maintain its integrity. The formulation of emulsions and foams wherein the external phase will not rupture during dispersion in said liquid or gas is discussed in many of the above patents.
However, later issued patents have shown some concern with breaking the emulsions used in the liquid membrane process so that the internal phase of such emulsions can be recovered and used in formulating fresh emulsion. (See for example U.S. Pat. No. 4,001,109.) In certain liquid membrane patents, the internal phase can be regenerated by heating with super heated steam to convert an impermeable species to a permeable species which is then re-extracted into a fresh solution. (See, for example, U.S. Pat. No. 4,029,744). In this process the impermeable species is the salt formed by the combination of a weak acid and a weak base which salt decomposes upon heating to regenerate the weak acid and the weak base in a noncombined state. This reaction is not an oxidation-reduction reaction as claimed for the instant process. Moreover, many emulsions are known to be generally unstable to heat of this nature, therefore this technique is not generally applicable to regenerating liquid membrane emulsions. The need to break the emulsion and regenerate and re-emulsify the internal phase for reuse are overcome by the process of the instant invention. Moreover regeneration without extensive heating of the emulsion is possible in one preferred embodiment of the instant invention described below.
It should also be noted that it is known in the art that reversible oxidation-reduction reactions may be induced by light and that such reactions may be carried out in the presence of artificial membranes, micelles, or oil-in-water microemulsions to inhibit back-reaction. See for example JACS, 100:20, Sept. 27, 1978, by Kiwi and Gratzel, at pp. 6314-6320, and the references cited therein. This reference does not suggest the use of "liquid membrane" processes in conjunction with light induced, reversible oxidation-reduction reactions, as disclosed and claimed herein. (See also 280 Nature pp. 823-8224, Aug. 30, 1979).